The United Nations said Wednesday investigators found 10 mass graves in the violence-wracked Kasai region in the Democratic Republic of Congo's centre.
"We have communicated to the (Congolese) government the presence of seven mass graves in the town of Demba ... and three mass graves in Tshimbulu," UN human rights officer Barbara Matasconi told reporters in capital Kinshasa.
The remote region has been plagued by violence since mid-August when government forces killed a tribal chief and militia leader, Kamwina Nsapu, who had rebelled against the central government of President Joseph Kabila.
Clashes between government forces and Nsapu supporters began in central Kasai, but the violence has since spilled over to the neighbouring provinces of Kasai-Oriental and Lomami, leaving at least 400 people dead.
Matasconi said her office has received "very serious" allegations of other mass graves but they have not been "confirmed".
The Congolese government previously admitted to the existence of three mass graves in the region, but provided no details on the identities of those killed or how they died.
Two foreign UN experts, an American and a Swedish-Chilean, along with their four Congolese staff were kidnapped on March 11 in the region. A Uruguayan peacekeeper was also shot and injured the week before that.
The United Nations has nearly 19,000 troops deployed in the DR Congo, its largest and costliest peacekeeping mission, with about 100 of those troops in the Kasai region.